Although there has been considerable effort spent investigating alternative electrochemical systems, the flooded electrolyte lead-acid battery is still the battery of choice for general purposes such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware, both industrial and military. These batteries may be periodically charged from a generator or other source of suitable DC power.
Historically, the electric power for such applications has been provided by conventional lead-acid batteries. The conventional lead-acid battery is a multi-cell structure, each cell generally comprising a set of vertical interdigitated monopolar positive and negative plates formed of lead or lead-alloy grids containing layers of electrochemically active pastes or active materials. The paste on the positive electrode plate when charged comprises lead dioxide (PbO2), which is the positive active material, and the negative plate contains a negative active material such as sponge lead. An acid electrolyte based on sulfuric acid is interposed between the positive and negative plates. The acid electrolyte, in effect, is a third active material in each cell of a lead acid battery and it, like the lead oxide anodic active material and the sponge lead cathodic active material, is reversibly changed during discharge of such a battery.
Bipolar batteries have recently gained attention and may serve to replace the use of the conventional battery in such applications due to their inherently decreased size and weight. Bipolar battery construction comprises a series of electrode plates that each contain a negative active material on one side and a positive active material on the other side, hence the terms "bipolar" and "biplate". The biplates are serially arranged in such a fashion that the positive side of one plate is directed toward the negative side of an adjacent plate. The bipolar battery is made up of separate electrolytic cells that are defined by biplate surfaces of opposing polarities. The biplates must be impervious to electrolyte and be electrically conductive to provide a serial electrical connection between cells.
Both conventional and bipolar lead acid batteries are characterized by a series of electrolytic cells. During the charging of these batteries, as well as through normal discharge, the water component of the electrolyte contained in each electrolytic cell is converted by electrolysis to form hydrogen gas and oxygen. The construction of some forms of these batteries permits the release of these gases by venting them to the atmosphere. Other forms of these batteries are valve regulated and are constructed so as to facilitate both the recombination of the oxygen gas and its reintroduction into the electrolyte solution. The electrolysis of the electrolyte during the charging of a vented battery produce a loss in the water constituent of the acid electrolyte, thereby causing the concentration of that acid and its specific gravity to increase and the liquid level to drop. Ideally, the concentration and specific gravity of a fully-charged flooded electrolyte lead-acid battery should be within a relatively narrow range of values for which the battery has been designed. An acid electrolyte of too-low concentration produces a decrease in battery performance, while an acid electrolyte of too-high concentration decreases the useful life of the battery and also reduces battery discharge performance. Therefore, in batteries where water loss can occur, it is necessary to periodically add water to the electrolyte to replenish the volume of electrolyte in the battery and to bring the specific gravity of the electrolyte into the design range from a too-high value. In order to permit the volumetric replenishment of the electrolyte, these batteries are typically constructed with a sealable opening at each cell which extends to an outside top surface of the battery. Accordingly, the user can replenish the electrolyte volume in each cell by adding water through each cell opening.
The technique of replenishing a battery electrolyte by adding water to each individual cell can be a dangerous, messy, time consuming, and inaccurate operation. When the user removes the cap of each cell during the replenishment operation, the user is exposed to both the battery electrolyte and the gases. The battery electrolyte is extremely acidic, and may cause burns to skin or permanent damage to clothing and the like. Therefore, contact by the user is to be avoided. Additionally, the gases produced during the charging or discharge of the battery is largely hydrogen which may be explosive under certain conditions.
During the replenishment operation it is not uncommon for the water being used to fill the battery electrolytic cell, to spill onto the surface of the battery or onto the user. The dangers associated with coming into contact with acidic battery electrolyte has already been described. However, when water is used to replenish the electrolytic cell the spilled water will oftentimes combine with concentrated electrolyte on the surface to form an acidic solution. The clean up of this spilled water may place the user in contact with the acidic solution, posing a risk of injury to the user. Additionally, improper watering practices, such as adding water to the electrolytic cells in a discharged battery, may result in electrolyte "flooding", due to the increase in electrolyte volume associated with the charging of the battery, again posing a risk to the user.
Replenishing each individual electrolytic cell with the proper amount of electrolyte is a matter of the user's judgement and requires that the user repeat the process of adding water and visually checking the level in each cell until the proper level is achieved. During the charging operation, the technique of visually checking the electrolyte level in each cell may pose a risk of electrolyte contact to the user due to generation of effervescent electrolyte caused by the gassing or electrolysis reaction of the water component of the electrolyte.
The construction of conventional and bipolar flooded electrolyte batteries also restricts the circulation and mixing of the battery electrolyte within each cell during the charging operation. Mixing of the electrolyte is important in order to ensure that each electrolytic cell comprises a homogeneous volume of electrolyte having a uniform specific gravity. Specific gravity is a measure the ability of the desired electrolyte to participate in the electrochemical reaction. Accordingly, an electrolytic cell having a homogenous electrolyte of selected volume and specific gravity is desirable because it will necessarily render an optimal amount of electrical energy and power and assure long life.
In conventional and bipolar electrolyte batteries, the agitation and mixing of electrolyte in each cell during the charging operation is accomplished by passing an amount of current usually measured as ampere-hours into the battery in excess of that required to restore the voltage capacity of the particular battery. This operation is referred to as "overcharging" the battery. During a normal charging operation, a current is passed into the battery for the purpose of reconstituting the active materials within the battery. The applied current reverses the electrochemical reaction responsible for the production of electrical energy during the proceeding discharge cycle, causing the reconstitution of the active materials. As the charging operation proceeds, the active material will continue to be reconstituted until the voltage and capacity of the battery is fully restored at which time the battery is said to be completely charged.
The additional current passed into the battery after it has been fully charged and reconstitution of the cell active materials (i.e., during the overcharge operation) no longer causes the reversal of the electrochemical reaction and reconstitution of the cell active materials, but will instead cause the water component of the electrolyte to electrolyze. The electrolysis of the electrolyte causes the gas products of that process (hydrogen and oxygen) to migrate through the volume of electrolyte as free bubbles rising to the surface of each electrolytic cell. The movement of the electrolysis gas bubbles through the electrolyte volume serves to agitate and mix the electrolyte within each electrolytic cell. As the overcharge operation continues, the agitation of the electrolysis gas bubbles produces a homogeneous volume of electrolyte having a uniform specific gravity. An electrolyte volume having a uniform specific gravity is desirable because it serves to maximize the electrical energy storage potential and life of each electrolytic cell. However, the operation of overcharging the battery in order to achieve a homogeneous volume of electrolyte generates heat and corrosion of the positive current collector which shortens the life of the battery, increases the need for electrolyte replenishment, increases the time and electrical energy consumed in charging the battery, and is economically inefficient.
It is therefore seen that a need exists for a flooded electrolyte battery conventional, bipolar or otherwise) which is constructed to facilitate the efficient replenishment of battery electrolyte in a manner that is not dangerous, messy, or time consuming and permits the user to easily and accurately replenish each electrolytic cell with the correct amount of electrolyte having the correct specific gravity.
A need exists for a flooded electrolytic battery (conventional, bipolar or otherwise) which is constructed to facilitate the mixing and homogenization of the battery electrolyte within each cell in a manner that avoids the need to overcharge the battery, and thus eliminates the adverse affects associated with the overcharge operation.